This invention relates generally to measuring apparatus and more particularly to an electronic scale device.
Various electronic scales are commercially available for providing a digital display of the weight of a body placed on a weighing pan. Many of the recently developed scales making use of integrated circuits and microcomputer controls for perfecting various scale operations such as weighing, counting, altering resolution, converting units, etc. In this regard, these devices must not only include electronic circuitry but must also include mechanical interfacing means between the electronic circuitry and the weighing pan.
The mechanical interface generally comprises some form of spring means for biasing the pan in a direction opposite to that of the gravitational force exerted upon the pan by the load being weighed. Thus, prior art devices in general include various types of spring means which generally undergo non-torsional deformation such as stretching or compression and accordingly, generally incur a heat rise in the compressed portions and a temperature drop in other areas. Temperature changes as such are undesirable under such circumstances because they produce corresponding changes in the dynamic properties of the spring, opposing its deflection. Accordingly, the heating and cooling effect introduces error into the measuring process unless compensating means (which are generally complex and expensive) are utilized.
It should be pointed out that with conventional strain gage scales, the above mentioned temperature change effects are not a problem since modern strain gages are constructed having temperature dependent properties which almost exactly offset the temperature dependent properties of the substrate spring material to which they are attached. Thus, the temperature effects are almost entirely cancelled out and accordingly, are generally invisible to the user of the device. However, capacitive or other displacement sensing devices, although otherwise generally superior to conventional strain gage scales, suffer the disadvantage of not having this built-in means for temperature compensation. In this regard, although this effect is generally small (e.g., between 0.01% and 0.3% of total deflection), it nonetheless becomes rather significant when making highly sensitive measurements.
Referring to the prior art, the U.S. Pat. No. 4,273,204 (Gillen), discloses a capacitive weighing scale utilizing a spring for biasing a movable capacitor plate against the force of the load being weighed. Although the Gillen device is generally suitable for its intended purpose, its spring comprises a bendable "C"-shaped steel member which has the tendency to produce the heretofore mentioned heating and cooling effect when it is used.
Other electronic scale devices disclosed in the prior art which also use non-torsional types of spring means are disclosed in U.S. Pat. Nos.: 4,158,395 (Brown); 4,308,929 (Estavoyer) and 4,309,929 (Batson).